According to, U. Tietze, Ch. Schenk, Halbleiterschaltungs-technik, 11. Auflage, 1. Nachdruck, Springer Verlag, Berlin 1999, Kapitel 22, Seiten 1189 und 1190 (U. Tietze, Ch. Schenk, Semiconductor Circuit Design, 11th edition, 2nd printing, Springer Publications, Berlin 1999, Chapter 22, pp. 1189-1190) (hereinafter, “Tietze”), it is often desired to first transform measured electrical signals before they are sent for example to an A/D converter in a processing unit. According to Tietze, measuring circuits are used which deliver an output signal supplied by a low-impedance voltage source. One way to measure the voltage of a high-impedance signal source without putting a load on the source is to use operational amplifiers as impedance converters. According to Tietze, attention is paid to the fact that the high-impedance input conductor line is sensitive to capacitative interference.
According to Tietze, Chapter 23.4, pp. 1256-1258, the sensor and the location at which the signals are evaluated are often separated by large distances and areas with high levels of interference. The amplifier that serves to amplify the measured signals, for example the impedance converter shown in Tietze, page 1189, FIG. 22.1, is therefore arranged in immediate proximity to the sensor.
According to WO 92/21962, there is a growing trend to use electrode systems, primarily glass electrodes, for the measurement of hydrogen ion concentrations in liquids, i.e. for pH measurements, for example to monitor chemical and biological processes in the field of food processing technology. The increasing use of electrodes for these purposes leads to increasingly stringent requirements in regard to the measurement accuracy in long-term use. Maintaining a satisfactory measurement accuracy can involve continuous monitoring of the condition of the electrodes being used, because the measurement accuracy could become compromised for example as a result of damage to the ion-sensitive membrane, contamination of the diaphragm, interruption of conductors and/or a short circuit within the electrode. The need to eliminate these performance-compromising factors as much as possible has led to a growing demand for methods of failure recognition which allow the condition and the proper functioning of the glass electrode to be monitored without interrupting the process in which the electrode is used and in particular without having to uninstall the glass electrode or to remove it from the medium being measured.
According to WO 92/21962, a measuring probe which includes a glass electrode and a reference electrode and which is immersed in a measurement medium is subjected to a square-wave pulse of variable amplitude and duration from a high-impedance source; the voltage of the measuring probe which has been changed by the probe impedance is measured and the measurement values are compared to a reference value for an intact measuring probe as determined by experiment or calculation. The square-wave pulses in this setup are presented at the analog output of a processor and are delivered to the measuring probe through a separate transmission line.
In a process described in EP 0 419 769 A2, the monitoring is carried out by symmetrical bipolar current pulses which are delivered by a control unit. The period length of the current pulses is freely selectable and can be set in accordance with the accuracy required for testing the probe. This method can involve a relatively extensive amount of circuitry, in particular two control lines which, for the generation of the symmetric bipolar current pulses, allow switching between a positive voltage source and a negative voltage source, or switching between the measurement phase for measuring the pH value and a test phase for testing the electrodes.
A method is disclosed in EP 0 497 994 A1 for testing a pH measuring electrode which in addition to the glass electrode and the reference electrode includes an auxiliary electrode. The disclosed concept further includes two processing devices which are supplied with an AC test voltage by a first and a second generator, respectively. The first generator in this arrangement works at a frequency that is an integer multiple of the frequency of the second generator. This allows separate monitoring of the glass electrode and the reference electrode. In the first case, the resistance of the chain formed of the glass electrode and the auxiliary electrode is tested, while in the second case the resistance of the chain formed of the reference electrode and the auxiliary electrode is tested. With the aforementioned frequency ratio between the generators, it is possible to achieve a sufficiently accurate differentiation between the output signals of the two processing units, as one output signal is suppressed in each case by the phase-sensitive rectification in the processing circuit of the other electrode. The processing devices therefore no longer directly detect the potential difference between the glass electrode and the reference electrode. They detect, however, a difference between the potentials of the glass electrode and the auxiliary electrode, and between the reference electrode and the auxiliary electrode. As both of the differences in potential are referenced to the same potential of the auxiliary electrode, the difference between the potentials of the glass electrode and the reference electrode can be determined by means of a differential amplifier. With this measuring circuit, the measuring probe therefore receives the AC test voltages of two different generators. These AC test voltages, in turn, are used for the subsequent phase-coherent processing of the signals and therefore also have to be transmitted normally from the processing unit to the measuring probe through appropriate conductor lines.
The use of additional conductor lines for the transmission of signals can involve added expense and complexity. Also, systems that are already installed can lack the conductor lines, and can either not be retrofitted or can be retrofitted with added cost and downtime of the system. With the trend towards miniaturization and the possibilities offered thereby for a decentralized arrangement of intelligent components, the desire for transmitting additional signals is on the increase, and more highly developed measuring probes that are designed for decentralized installations may only be of limited use in existing systems.